feat(05-03): TicketComment model and portal ticket service

- Add TicketComment model for conversation threads (append-only)
- Add paymentInstructions field to TenantSettings
- Add ticketComments relation to User model
- Create portal-ticket-service with ensurePortalUser shadow User pattern
- Implements createPortalTicket, listPortalTickets, getPortalTicket, addTicketComment
- Portal ticket creation delegates to existing createTicket with source=SUBSCRIBER

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -272,6 +272,8 @@ model TenantSettings {
   autoSuspendDays  Int      @default(30)
   /// Days before billing date to generate prepaid invoices (default 7)
   prepaidLeadDays  Int      @default(7)
+  /// Freeform payment instructions shown on the portal "pay online" page (e.g., GCash, bank details)
+  paymentInstructions String?
   createdAt        DateTime @default(now())
   updatedAt        DateTime @updatedAt
 
@@ -448,6 +450,8 @@ model User {
   createdExpenses   Expense[]       @relation("ExpenseCreatedBy")
   /// Expenses approved by this user
   approvedExpenses  Expense[]       @relation("ExpenseApprovedBy")
+  /// Ticket comments authored by this user
+  ticketComments    TicketComment[]
 
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
@@ -695,6 +699,27 @@ model Ticket {
   @@index([subscriberId])
 
   jobOrders JobOrder[]
+  comments  TicketComment[]
+}
+
+/// A TicketComment is a message in a ticket conversation thread.
+/// Subscribers and staff can both add comments to open tickets.
+/// Comments are append-only — no edits or deletes.
+model TicketComment {
+  id           String   @id @default(uuid())
+  tenantId     String
+  ticketId     String
+  ticket       Ticket   @relation(fields: [ticketId], references: [id])
+  /// Who posted this comment — can be subscriber (CLIENT) or staff
+  authorId     String
+  author       User     @relation(fields: [authorId], references: [id])
+  /// For portal comments, link to the subscriber directly
+  subscriberId String?
+  message      String
+  createdAt    DateTime @default(now())
+
+  @@index([tenantId])
+  @@index([ticketId])
 }
 
 /// A Collection records cash received from a subscriber by a field collector.